1. Field of the Invention
The present invention relates to a multi-band antenna system including a retractable whip antenna and a meander antenna having a plurality of selectively coupled meander radiating elements formed on a dielectric flexible board. The meander antenna may include one or more passive elements which may be selectively coupled to the meander radiating elements of the meander antenna.
2. Description of the Related Art and the Relationship of the Instant Invention Thereto
In the rapidly evolving technology of cellular communication, there is an emerging thrust on the design of multi-purpose cellular handsets. A cellular handset which has system capabilities of both dual cellular and non-cellular (such as GPS) applications has become a new feature. Thus, there is a growing trend to design antennas which operate in both the dual cellular and non-cellular frequency bands. The inherent problem facing such a design is the bandwidth requirement at the upper resonance of the antenna to simultaneously cover both the GPS band (1575 MHz) and the upper cellular band such as either DCS (1710-1880 MHz) or PCS (1850-1990 MHz). The combined bandwidth requirement to cover the GPS and PCS bands of operation approximates about 23.35%. The easy recourse of an additional antenna with a separate feed to cover the GPS band alone has not proved to be an attractive alternative. In view of this, a single feed multi-band antenna operating both in the dual cellular and non-cellular bands is a topic of considerable importance for cellular applications. The instant invention is a new method of designing a single feed multi-band retractable antenna operating in the dual cellular bands (AMPS/PCS) as well as non-cellular (GPS) band. The significant aspect of this invention pertains to the design of the single feed, multi-element meander antenna as the primary radiator in the retracted position of a multi-band whip antenna. In this invention, a multi-element meander antenna or radiator replaces the conventional helical coil radiator to constitute the primary radiator for the retracted position of a multi-band whip antenna.
A conventional prior art multi-band retractable antenna 100 for a cellular handset 101 is shown in FIGS. 16A and 16B. FIG. 16A illustrates the multi-band retractable antenna in its retracted position. A plastic housing or sheath 102 fully encloses a helical coil radiator or a meander radiator positioned therein. The plastic housing 102 is usually mounted near one of the corners at the top edge 103 of the handset 101. The plastic housing 102 with a helical coil radiator or meander radiator therein is usually positioned so as to have an outward extension with respect to the top edge 103 of the handset 101. Such a position is conducive for good antenna radiation characteristics. In the retracted position of the multi-band retractable antenna, 100, as depicted in FIG. 16A, the whip antenna 104 with stopper 105 mounted thereon is decoupled from the helical coil radiator or meander radiator positioned within the plastic housing 102. Only the radiator inside the plastic housing 102 is allowed to retain contact with the RF connector 106 placed on the chassis 107 of the handset 101. In the retracted position of the multi-band antenna 100, the helical coil radiator or meander radiator alone is the dominant or primary radiator with an insignificant contribution of the whip antenna 104.
FIG. 16B illustrates the configuration of the prior art multi-band antenna 100 in its extended position. In this configuration, the whip antenna 104 is pulled up and through the connector 106 with the stopper 105 of the whip antenna 104 making contact with the RF connector 106. In the extended position, along with the whip antenna 104, the helical coil radiator or meander radiator positioned within the plastic housing 102 is also connected to the RF connector 106. When the whip antenna 104 is in the extended position, the dominant radiator of the retractable multi-band antenna 100, however, is the linear whip antenna 104 with its length designed at least for the quarter wavelength of operation and extending well above the plastic housing 102. It is of importance to note that the coupling between the whip antenna 104 and the helical coil radiator or meander radiator requires optimization to obtain the desired radiation characteristics of the whip antenna.
In most conventional multi-band retractable antenna designs, the dominant or primary radiator in the retracted mode is usually an ordinary helical coil. With a single coil of simple geometry, realizing a multi-band operation with satisfactory bandwidth imposes the requirement of an external matching network. If the desired frequency bands of operation include more than two bands, e.g. AMPS/GPS/PCS or GSM/GPS/DCS, the design of the helical coil is an involved task. Such a multi-band retractable antenna design may result in a complicated helical coil which is difficult to fabricate. Therefore, the design of a multi-band radiating element which is easy to fabricate is desirable. In the proposed invention, resorting to the meander radiator planar technology, a radiator in the form of a plurality of meander radiating elements is designed and etched on a dielectric flexible board resulting in fabrication ease. Unlike the design of a conventional helical coil, the design of the meander radiator on the flexible board does not impose any constraint on the complexity of the antenna structure from a fabrication point of view. Any arbitrary variations in the profiles of the radiating elements of the meander radiator on the flexible board can be easily and consistently reproduced with relative ease. This is a distinct advantage of the choice of the meander radiator over conventional helical coils as the primary radiator in the retracted position of multi-band retractable antennas.
In the design of a retractable antenna, the input impedance of the whip (wire) antenna (normally of quarter wavelength or more in its length) is different from the desirable 50 ohms. The deviation of the input impedance from the desired nominal impedance of 50 ohms depends mainly on the chosen length for the whip antenna as well as the chassis or associated ground plane of the radio device. To realize the impedance match at the RF input port of the radio or communication device, an external matching circuit with discrete inductors and capacitors is common in most of the prior art designs. Apart from the external matching network for the extended position, a separate and additional external matching network for the impedance match for the radiator in the retracted position may also be needed. Such a necessity arises to obtain the impedance match of the helical coils (which are the dominant radiators in the retracted mode) at the RF input port of the device. Therefore alternate designs of multi-band retractable antennas devoid of either the single or dual external matching networks are of significant importance for cellular communication. This invention proposes the design of multi-band retractable antennas without necessitating the requirements of impedance matching networks either for the extended or the retracted positions. In this invention, the meander radiator is designed for a self-impedance match in the retracted position. In addition, the meander radiator is also designed to serve the analogous role of an external matching network to realize the impedance match for the whip antenna in the extended position of the multi-band retractable antenna. The proposed invention circumvents the necessity of an external matching network to realize the design of a single feed multi-band retractable antenna whose upper resonant band itself comprises multiple frequency bands with wider separation between them such as GPS/PCS bands.
In the recent past, there is an emerging trend for a closer look at the impedance characteristics of antennas toward optimizing gain performance thereof. The current concept of emphasizing the antenna VSWR, alone, for the satisfactory gain performance is changing. In many antenna designs, the gain performance has greater dependence on the relative magnitudes of the resistive and reactive components of the antenna impedance rather than on the mere magnitude of VSWR alone. Therefore the multi-band antenna designs with versatile means of controlling its impedance characteristics is of special relevance to cellular communication applications.
The choice of the meander radiator as the primary radiator in the retracted position of the proposed multi-band retractable antenna provides the designer additional degrees of freedom hitherto not normally found in the design of conventional retractable antennas with simple helical coils. The present invention proposes several schemes for the design of a single feed multi-band meander radiator either with a combination of active elements only, or, with a combination of active and passive elements. Deviating distinctly from the prior art designs, this invention presents design schemes for the single feed multi-band meander radiator which utilizes the combination of selective coupling and multiple element parasitic effects between active and passive radiators.
U.S. Pat. No. 6,069,592 (xe2x80x9cMeander Antenna Devicexe2x80x9d by Bo Wass of Aligon AB, Sweden) deals with meander antennas for dual or multi-band operation for the retracted position of a whip antenna. Similar to the proposed design of this invention, the radiator for the retracted position of the multi-band whip antenna suggested in the above patent also claims two separate meander radiating elements resonating in the respective lower and upper frequency bands. The distinct difference between the above patent and the proposed invention lies in the relative orientation and configuration of the meander radiating elements for optimizing the performance of the multi-band radiator for the retracted position of the whip antenna. Unlike the patent by Wass, the dual or multiple meander radiating elements of this invention provide for the protrusion of one meander radiating element (designed for a particular resonant band) into the other meander radiating element providing a distinctly different frequency band. Such an intentional protrusion results in the selective coupling between the two meander radiating elements operating in different frequency bands. For the design of a multi-band meander antenna in the retracted position of the whip antenna with only two meander radiating elements, the profiles of the meander radiating elements of this invention are chosen such that the closed loops of one meander radiating element protrude into the open loops of the other meander radiating element resulting in coupling therebetween. For the design of multi-band meander antenna with three elements of this invention, the central element includes the provision for the attachment of coupling stubs to it. The coupling stubs on the central element are designed to protrude into the open loops of an adjacent meander radiating element resulting in selective coupling between different meander radiating elements designed for different resonant frequencies.
Another distinction between the patent by Wass and the proposed invention is in the design of the third (central) element thereof. In Wass"" patent pertaining to the design of the multi-band radiator with three elements, the third element is similar to the first and second meander radiating elements, but tuned to a third frequency different than the first and second resonant frequencies. From this, it is clear that the design configuration of Wass has the third meander radiating element connected to the other two meander radiating elements by a common feed line. This in turn implies that the three meander radiating elements of Wass"" invention are active elements connected together to a common feed point for multi-band operation. In the proposed design of the multi-band meander antenna with three elements of this invention, there is no such restriction on the third (central) element. This invention proposes a single feed multi-band meander antenna whose configuration can be a combination of active and passive elements as well. In some of the embodiments of this invention, the third (central) element can be a parasitic radiator. Such a parasitic central element is physically isolated from the other adjacent meander radiating elements. Further, unlike the case of Wass"" patent, this invention proposes several schemes wherein the third (central) element need not be similar to the other two adjacent elements in its profile or shape. The central element of this invention can be substantially linear as compared to the conventional zigzag profiles of the other two adjacent radiating elements. Unlike the patent by Wass, this invention proposes the design of the combination of a plastic housing which encloses the multi-band meander antenna and the associated metal connector for providing the RF feed path to the antenna as a single, over-molded part. Such a choice improves the cost effectiveness of fabrication and simplifies the integration of the antenna to the radio device.
Some of the design embodiments of a single feed multi-band multi-element meander antenna of this invention also have the advantage of improved cross-polarization performance, which often can be a desirable feature. The significant improvement in the cross-polarized radiation patterns without noticeable degradation of the co-poarized radiation characteristics will improve the cellular antenna performance in its User position.
This invention proposes several embodiments of providing a single feed multi-band meander antenna or radiator with dual and multiple elements as the primary radiator for the retracted position of the multi-band retractable antenna. The design of the multi-band meander radiator of this invention as a radiator for the retracted position of whip antenna accomplishes the requisite bandwidth for tri-band (AMPS/PCS/GPS) performance without the need for an external matching network. The absence of the requirement of an external matching network is valid for both the extended and retracted positions of the multi-band whip antenna while still maintaining the tri-band operation of AMPS/PCS/GPS bands. The dual or multiple radiating elements of the meander radiator of this invention permit the protrusion of one meander radiating element (designed for a particular resonant band) into the other meander radiating element supporting a distinctly different frequency band. Such an intentional protrusion results in the selective coupling between the two meander radiating elements operating in different frequency bands. To characterize the bandwidth and gain performance with varying structural modifications, the design of the central radiating element with and without coupling stubs is also described. In particular, the coupling stubs of the central element protrude into the open loops of the meander radiating element designed for the resonant lower band. The effect of varying the position of the contact point of the central element on a line that is common to the other two adjacent meander radiating elements is also provided for in this invention. In another embodiment of this invention, instead of the central element making a direct physical contact with the other meander radiating elements placed on either side of the central element, the (third) central meander element is designed to have physical separation from the adjacent meander radiating elements leading to its functioning as a parasitic element. Such a central element of a parasitic nature is designed with or without the above-referred coupling stubs protruding into the open loops of the meander antenna designed for lower resonant band. The relative merits for the choice of the central radiating element either as an active element or passive (parasitic) element have also been addressed in this invention. The advantages of having a design variation in the shape of the central parasitic element (either Inverted L-shape or Inverted U-shape) have also been studied in this invention.
In the first embodiment of this invention, a design of the multi-band meander antenna 10 (with only two radiating elements) as a primary radiator for the retracted position of the whip antenna, the profiles of the meander radiating elements are chosen such that the closed loop of one meander radiating element (designed for a resonant frequency) directly protrude into the open loop of the other meander radiating element (designed for a different resonant frequency) resulting in selective coupling between them. The realizable selective coupling can be optimized to control/improve the overall bandwidth and radiation performance in the extended and retracted positions of the multi-band whip antenna. In the second embodiment of this invention dealing with the design of multi-band meander antenna 20 with three elements, the central element includes coupling stubs. The coupling stubs are designed to protrude into the open loops of an adjacent meander radiating element resulting in selective coupling between different meander radiating elements. The variation in the selective coupling is determined by the location of the coupling stubs on the central element, the shape of the coupling stubs and the extent of the protrusions of the coupling stubs into the open loops of the adjacent meander radiating element designed for a different resonant frequency.
In the second embodiment, the conjuncture point connecting the third (central) element to the other elements is in close proximity to the open loops of the meander radiating element designed for the upper resonant frequency. In the third embodiment of this invention dealing with the design of single feed multi-band meander antenna 30 with three elements, the common (conjuncture) point connecting the third (central) element to the other two elements is positioned nearer to the open loops of the meander radiating element designed for the lower resonant frequency. A relative comparison between the results of the second and third embodiments of this invention illustrates the effect of the relative proximity of the conjuncture point of the third element to the open loops of the other radiating elements.
In the fourth embodiment of this invention, the design configuration of the single feed multi-band meander antenna 40 involves the combination of active and passive elements. Unlike the second and third embodiments of this invention, the third or central element is designed as a passive radiator to serve as a parasitic to the adjacent active meander radiating elements designed for the lower and upper resonant frequencies of interest. The central element having an inverted U-shape is physically isolated from the other two adjacent meander radiating elements. The central element having an inverted U-shape has the coupling stubs protruding into the open loops of the meander radiating element designed for lower resonant frequency of multi-band operation. The fourth embodiment of this invention demonstrates the possibility of invoking the combination active and passive elements in the design of single feed multi-band meander radiating element with satisfactory bandwidth to cover (AMPS/GPS/PCS) bands. A comparative study of the results of the second and third embodiments with that of the fourth embodiment of this invention illustrates the effect of the choice of the active or passive third element on the resonant and gain characteristics of the multi-band meander radiating element.
The single feed multi-band meander antenna 50 of the fifth embodiment of this invention differs from the fourth embodiment in the shape of the third (central) element acting as a parasitic element to the other radiating elements. In this embodiment also, the third element is designed to be a passive radiator to act as a parasitic element. Instead of an inverted U-shape as in the fourth embodiment, the third element of the fifth embodiment of this invention has the shape of an inverted L-shape. The central element of inverted L-shape has coupling stubs protruding therefrom into the open loops of the meander radiating element designed for lower resonant frequency of multi-band operation. The influence of the shape of the passive third element on the bandwidth and the radiation performance of the multi-band meander radiating element can be inferred through a comparative study of the results of the fourth and the fifth embodiments of this invention.
The single feed multi-band meander antenna 60 of the sixth embodiment of this invention differs from the meander antenna 50 of the fifth embodiment in the configuration of the third (central) element acting as a parasitic element to the other radiating elements which are designed for the resonance at the lower and upper cellular bands. In the sixth embodiment of this invention also, the third element is configured as a passive element and functions as a parasitic element to the other radiating elements. The absence of the coupling stubs on the parasitic central element of meander antenna 60 of the sixth embodiment of this invention distinguishes it from the meander antenna 50 referred in the fifth embodiment. The relative comparison of the results of fifth and the sixth embodiments of this invention offers an insight into the influence of the coupling stubs of the parasitic central element on the bandwidth as well as the radiation characteristics of the multi-band meander antennas 50.
The meander antenna 70 of the seventh embodiment of this invention differs from the meander antenna 60 of the sixth embodiment in the shapes of the parasitic third (central) element. The parasitic third element of the meander antenna 70 is of an inverted U-shape instead of an inverted L-shape as in meander antenna 60. The comparative study of the results of the sixth and the seventh embodiments of this invention enables to characterize of influence of the shape of the third element (without coupling stubs) on the bandwidth and the radiation characteristics of the multi-band meander antennas 60 and 70.
The design embodiments of the single feed multi-band meander antennas of this invention for the retracted position of the whip antenna have the advantage of compactness and fabrication ease. The planar technology of meander antennas of this invention also has the advantage of improved production tolerance resulting in reduction of rejection rate. All the multiple elements of the proposed multi-band meander antenna can be formed in a single process of etching or printing. Therefore the proposed multi-band meander antenna with multiple elements formed on flexible board of this invention is amenable for large-scale production and is cost-effective to manufacture. The design of the single feed multi-band multi-element meander antenna of this invention is versatile and has a greater degree of freedom to control its impedance characteristics. Many design options yielding almost the same results are possible with the proposed design. In view of the emerging demand of a single antenna for the cellular handset with multi systems application capabilities, this invention has a greater emphasis on the design of multi-band retractable antenna for tri-band operation comprising the AMPS band (cellular) for its lower resonance and the combined PCS (cellular) and GPS (non-cellular) band for its upper resonance. This invention also accomplishes the realization of adequate bandwidth of the multi-band retractable antenna comprising the whip antenna and the multi-element meander antenna without resorting to either single or dual external impedance matching networks. The gain performance of the multi-band meander antennas proposed in this invention is better than that is usually associated with the conventional helical coil design.
One of the principal objectives of this invention is to provide a single feed multi-band meander antenna for the retracted position of the whip antenna to cover dual cellular and non-cellular frequency bands. Specifically, one of the primary objectives of this invention is to provide a single feed multi-element meander antenna for multi-frequency operation whose upper resonance comprises the two frequency bands with wider separation between them.
Another objective of this invention is to provide a design scheme for realizing the satisfactory bandwidth of a multi-band retractable antenna devoid of external impedance matching networks in both its extended and retracted positions.
Another objective of this invention is to provide a design scheme for single feed multi-band retractable antennas with better and increased provisions to control the impedance characteristics thereof.
A further objective of this invention is to provide a multi-band meander antenna or radiator as a retracted position radiator with a desirable feature of improving or controlling the cross-polarization performance of the retractable antenna.
An objective of this invention is also to characterize the performance of a single feed multi-band multi-element meander antenna whose configuration consists of a combination of active and passive elements
One of the objectives of this invention is the shape optimization of the active or passive central element of a single feed multi-band multi-element meander antenna to improve the overall performance of the retractable antenna in its retracted and extended positions.
Yet another objective of this invention is to provide a single feed multi-element multi-band meander antenna or radiator, for the retracted position, that takes advantage of features for structural simplicity, compactness of size and fabrication ease toward high volume manufacturing.
An important objective of this invention is to provide the combination of a plastic housing encompassing the multi-element multi-band meander antenna as well as the associated RF connector as a single over-molded part to simplify and enhance the ease of antenna integration to the communication device.
These and other objectives will be apparent to those skilled in this art.